Two-stroke engine with storage duct

Abstract

A two-stroke engine in a hand-held working tool such as a motor chainsaw has a combustion chamber delimited by a piston which drives a crankshaft. An outlet for removing exhaust gases from the combustion chamber and an intake for supplying fresh mixture to the combustion chamber are provided. A storage duct opens into the intake and the crankcase and is connected with a carburetor. Combustion air flows via a crankcase intake and a transfer passage into the combustion chamber. For providing good power development with good exhaust emission values under all operating conditions, the flow cross-section of the mixture-forming channel leading to the storage duct is smaller by more than one half than the flow cross-section of the intake passage leading to the crankcase intake. During idle, the fuel required for operation is supplied substantially exclusively via the mixture-forming channel leading to the storage duct.

What is claimed is:  
     
       1. A two-stroke engine as a drive motor in a hand-held working tool, the two-stroke engine comprising: 
       a cylinder ( 2 ) and a reciprocating piston ( 5 ) arranged in the cylinder ( 2 ), wherein the reciprocating piston ( 5 ) delimits a combustion chamber ( 3 ) within the cylinder ( 2 );  
       a crankshaft ( 7 ) rotatably supported in a crankcase ( 4 );  
       a connecting rod ( 6 ) connected to the reciprocating piston ( 5 ) and the crankshaft ( 7 ), wherein the piston ( 5 ) drives the crankshaft ( 7 ) via the connecting rod ( 6 );  
       wherein the cylinder ( 2 ) has an outlet ( 10 ) for removing exhaust gases from the combustion chamber ( 3 ) and a cylinder intake ( 11 ) supplying a fresh mixture into the combustion chamber ( 3 );  
       a storage duct ( 14 ) having a first end ( 13 ) formed by the cylinder intake ( 11 ) and having a second end ( 15 ) opening into the crankcase ( 4 );  
       a first mixture forming device ( 8   a ) for forming a fuel/air mixture having a first mixture-forming channel ( 28 );  
       a second mixture forming device ( 8   b ) having a second mixture-forming channel ( 17 );  
       wherein the first mixture-forming channel ( 28 ) is connected to the storage duct ( 14 ) between the first and second ends ( 13 ,  15 );  
       wherein the crankcase ( 4 ) comprises a crankcase intake ( 16 ), connected to the second mixture-forming channel ( 17 ), for taking in combustion air and fuel;  
       a first fuel path ( 41 ) opening into the first mixture-forming channel ( 28 ) and a second fuel path ( 31 ) opening into the second mixture-forming channel ( 17 );  
       a transfer passage ( 18 ) having a first end, connected to the combustion chamber ( 3 ) and having a transfer window ( 19 ) opening into the combustion chamber ( 3 ), and a second end ( 20 ) connected to the crankcase ( 4 );  
       wherein the first mixture-forming channel ( 28 ) leading to the storage duct ( 14 ) has a flow cross-section (d) that is smaller by more than one half than a flow cross-section (D) of the second mixture-forming channel ( 17 ) leading to the crankcase intake ( 16 ); and  
       wherein, during idle of the internal combustion engine ( 1 ), the second fuel path ( 31 ) opening into the second mixture-forming channel ( 17 ) is substantially completely closed and fuel, required for operation, is supplied substantially exclusively via the first mixture-forming channel ( 28 ) leading to the storage duct ( 14 ).  
     
     
       2. The two-stroke engine according to  claim 1 , wherein, during idle of the internal combustion engine ( 1 ), combustion air is supplied substantially exclusively via the first mixture-forming channel ( 28 ) while the crankcase intake ( 16 ) is substantially closed. 
     
     
       3. The two-stroke engine according to  claim 1 , wherein, at least in one of partial load operation and full load operation, fuel supplied to the internal combustion engine ( 1 ) is supplied in a subordinate proportion via the second mixture-forming channel ( 17 ) and the crankcase intake ( 16 ). 
     
     
       4. The two-stroke engine according to  claim 3 , wherein fuel supplied to the combustion engine ( 1 ) is supplied in a proportion of approximately 0% to 35% via the second mixture-forming channel ( 17 ) and the crankcase intake ( 16 ). 
     
     
       5. The two-stroke engine according to  claim 1 , wherein the ratio (V=d/D) of the flow cross-section (d) of the first mixture-forming channel ( 28 ) leading to the storage duct ( 14 ) relative to the flow cross-section (D) of the second mixture-forming channel ( 17 ) leading to the crankcase intake ( 16 ) is in a range of ½ to {fraction (1/12)}. 
     
     
       6. The two-stroke engine according to  claim 1 , wherein the first mixture-forming channel ( 28 ) has a mixture-forming channel throttle ( 40 ) configured to change the flow cross-section (d) of the first mixture-forming channel ( 28 ). 
     
     
       7. The two-stroke engine according to  claim 6 , wherein the second mixture-forming channel ( 17 ) has an intake passage throttle ( 29 ,  30 ,  49 ) configured to change the flow cross-section (D). 
     
     
       8. The two-stroke engine according to  claim 7 , wherein the mixture-forming channel throttle ( 40 ) and the intake passage throttle ( 29 ,  30 ,  49 ) are coupled position-dependently with one another. 
     
     
       9. The two-stroke engine according to  claim 8 , wherein the mixture-forming channel throttle ( 40 ) and the intake passage throttle ( 29 ,  30 ,  49 ) are coupled with opposite orientation such that the first mixture-forming channel ( 28 ) is open and the second mixture-forming channel ( 17 ) is closed during idle and the mixture-forming channel ( 28 ) is throttled and the second mixture-forming channel ( 17 ) is open under full load operation. 
     
     
       10. The two-stroke engine according to  claim 9 , wherein the mixture-forming channel throttle ( 40 ) and the intake passage throttle ( 29 ,  30 ,  49 ) are selected from the group consisting of a throttle blade ( 30 ,  40 ), a rotary sleeve valve ( 29 ), and a throttle slide ( 49 ). 
     
     
       11. The two-stroke engine according to  claim 1 , wherein the flow cross-section (d) of the first mixture-forming channel ( 28 ) is unchangeably fixed. 
     
     
       12. The two-stroke engine according to  claim 1 , wherein the flow cross-section (D) of the second mixture-forming channel ( 17 ) is unchangeably fixed. 
     
     
       13. The two-stroke engine according to  claim 1 , wherein one of the first and second fuel paths ( 341 ,  41 ) is a controllable fuel path. 
     
     
       14. The two-stroke engine according to  claim 1 , wherein the controllable fuel path ( 41 ) is mechanically controllable. 
     
     
       15. The two-stroke engine according to  claim 14 , wherein the controllable fuel path comprises a valve ( 42 ). 
     
     
       16. The two-stroke engine according to  claim 15 , wherein the valve is a needle valve ( 43 ). 
     
     
       17. The two-stroke engine according to  claim 1 , wherein the first and second mixture-forming channels ( 17 ,  28 ) are configured to aspirate fuel by vacuum. 
     
     
       18. The two-stroke engine according to  claim 1 , wherein the first and second mixture-forming channels ( 17 ,  28 ) are arranged in a common carburetor housing. 
     
     
       19. The two-stroke engine according to  claim 18 , wherein the first and second mixture-forming channels ( 17 ,  28 ) extend substantially parallel. 
     
     
       20. The two-stroke engine according to  claim 19 , wherein the first and second mixture-forming channels ( 17 ,  28 ) are preferably positioned atop one another in a direction of a longitudinal cylinder axis ( 21 ) of the cylinder ( 2 ). 
     
     
       21. The two-stroke engine according to  claim 20 , wherein the first mixture-forming channel ( 28 ) is connected by a check valve ( 24 ) with the storage duct ( 14 ).